Age-related macular degeneration (AMD) is the number one cause of blindness for the elderly population over 60 years of age. It is a devastating disease that destroys central vision in the affected individuals, robbing them of their ability to perform activities necessary for everyday life such as reading and driving (Bressler et al., 1988; Evans, 2001; Gottlieb, 2002). In one study, the prevalence of AMD in persons 75 or older has been reported to be 7.8% (Klein et al., 1992).
AMD is a slow, progressive disease that involves cells of the outer retinal layers (including photoreceptors and the retinal pigment epithelial (RPE) cells that support the photoreceptors), as well as cells in the adjacent vascular layer of the eye known as the choroid. Macular degeneration is characterized by the breakdown of the macula, a small portion of the central retina (about 2 mm in diameter) responsible for high-acuity vision. Late-onset macular degeneration (i.e., AMD) is generally defined as either “dry” or “wet.” The wet (“exudative”) neovascular form of AMD affects approximately 10% of those with the disease, and is characterized by abnormal blood vessels growing from the choriocapillaris through the RPE, typically resulting in hemorrhage, exudation, scarring, and/or serous retinal detachment. Approximately 90% of patients with AMD have the non-neovascular dry form, characterized by atrophy of the RPE and loss of macular photoreceptors.
One of the clinical hallmarks of AMD is the presence of deposits of debris-like material, termed “drusen,” that accumulate on Bruch's membrane, a multilayered composite of extracellular matrix components separating the RPE (the outermost layer of the retina) from the underlying choroid. Drusen can be observed by funduscopic eye examination. These deposits have been extensively characterized in microscopic studies of donor eyes from patients with AMD (Sarks, et al., 1988). The deposits observed in the living eye upon clinical examination are classified as either soft drusen or hard drusen, according to several criteria including relative size, abundance, and shape of the deposits (reviewed, for example, in Abdelsalam et al., 1999). Histochemical and immunocytochemical studies have shown that drusen contain a variety of lipids, polysaccharides, glycosaminoglycans and proteins (Abdelsalam et al., 1999; Hageman et al., 1999, 2001).
There is presently no cure for AMD. Several types of treatments are available, with laser photocoagulation of abnormal vessels in the wet form of the disease being the standard (Gottlieb, 2002; Algvere and Seregard, 2002). This treatment is limited by the fact that only well-delineated neovascular lesions can be treated in this way and that 50% of patients will suffer recurrence of the leakage from the vessels (Fine et al., 2000). Because of the energy of the laser required for this treatment, the photoreceptors in the treated area will also die, and the patient will also often suffer central blindness immediately after the treatment. New neovascular lesions will eventually develop, requiring repeated treatments.
Photodynamic therapy, which combines low energy laser activation with a photosensitive agent, has been a valuable addition to the laser treatment approach (Bressler, 2001). In this method, a photosensitive agent, i.e., verteporfin is used which has an affinity for abnormal new blood vessels. Selective targeting of these vessels can be activated by nonthermal laser to produce reactive oxygen species which can destroy the abnormal vessels. In a study group, only 33% of those receiving photodynamic therapy with verteporfin had substantial loss of vision, compared to 61% of those who did not receive verteporfin. The treatment, however, was only beneficial for patients with classic choroidal neovascular membranes. The full long-term benefit of this new treatment modality has yet to be established. Despite this advance, however, the treatment does not prevent the subsequent formation of new neovascular lesions.
Other available treatments for the wet form of AMD include submacular surgery and external-beam radiation therapy. Those under study include retinal translocation and inhibition of vascular endothelial growth factor (Algvere and Seregard, 2002). For prevention of progression to advanced AMD, treatment with antioxidants, including vitamins C and E, β-carotene, and zinc, was shown to be helpful, and prophylactic laser treatment is under study (Gottlieb, 2002).
Despite the above-described advances, it is recognized that current treatment for AMD is mostly palliative (Algvere and Seregard, 2002). None of the available treatments attacks the fundamental cause of the disease, which is unknown. The disease therefore can continue to progress following treatment, with re-development of neovascularization and destruction of the macula. Accordingly, there remains a compelling need to understand the molecular mechanism of this disease, so that therapeutic treatment or cure can be directed at its root cause.
It is well recognized that genetic factors play an important role in the etiology of AMD. For example, it has been reported that people with a family history of AMD and siblings of AMD patients have a higher risk of developing AMD (Evans, 2001). Monozygotic twins have shown a higher concordance rate of clinical features of AMD compared to dizygotic twins (Klein et al., 1994). Another study found all monozygotic twins affected with AMD to be concordant for AMD while only 42% of dizygotic twins were concordant (Meyers et al., 1995). Accordingly, one major approach to understanding AMD etiology is to look for genes involved in AMD. For example, approaches such as linkage analysis in large families, allele sharing analysis among sib pairs, and association studies in populations have been used in attempts to identify genes associated with AMD (Guymer, 2001). Linkage to chromosomal region 1q was reported in a large AMD family (Klein et al., 1998). Results of an allele sharing analysis did not yield any new candidate genes (Weeks et al., 2000). An association of a mutation in hemicentin-1 has been reported in a familial form of age-related macular degeneration mapping to human chromosome 1q in a large family (Schultz et al., 2003).
Another genetic strategy for AMD is to test genes causing other forms of inherited macular degenerations as putative causative genes (“candidate genes”) for AMD. Several macular diseases with a clearly hereditary pattern of inheritance (so-called “Mendelian macular degenerations”) have been described that resemble AMD in phenotype. Examples of these diseases include Sorsby's fundus dystrophy, Stargardt's disease, Best disease, and Doyne's honeycomb retinal dystrophy (Guymer, 2001). Causative genes for these diseases have been analyzed as candidate genes for AMD. To date however, none of them has clearly demonstrated a causal relationship with AMD. For example, the ATP-binding cassette transporter gene (ABCR) was found to be the pathogenic gene for recessive Stargardt's disease (Hutchinson et al., 1997). ABCR was proposed as a candidate gene for AMD, and in one study, 16% of patients with AMD were initially shown to have mutations in this gene (Allikmets et al., 1997). This conclusion, however, has been challenged (Stone et al., 1998).
The most likely reason for the failure to find AMD genes through classical genetic approaches such as chromosomal mapping, genetic linkage analysis, and candidate gene analysis, is that AMD is a “multigene,” or “complex” genetic disease. Complex genetic diseases are those diseases believed to be caused by changes in multiple genes. Such diseases characteristically demonstrate a complex pattern of inheritance (Heiba et al., 1994; Klein et al., 1994). In the case of AMD, a disease of old age, it is generally thought that the course of the disease is influenced not only by the combined effects of the above-described multiple genetic factors, but is further affected by certain environmental risk factors.
A second broad approach aimed at discovering causative genes in AMD has been hypothesis-based research aimed at elucidating the mechanism of the disease, with the goal of secondarily identifying the genes involved in the mechanism. Numerous hypotheses regarding the pathogenic mechanism of AMD have been proposed and tested, resulting in a voluminous literature on this subject.
Oxidative damage has been one major theme as a proposed mechanism for AMD (Winkler et al., 1999; Evans, 2001; Husain et al., 2002). The retina is known to have an extremely high consumption of oxygen, and the photoreceptors and RPE are in a very oxygen-rich environment. The RPE is situated immediately adjacent to the choriocapillaris, a rich capillary plexus coursing with highly oxygenated blood. The retina is a light-sensitive organ in which photoactivated events are constantly occurring during times of light exposure, resulting inter alia in the production of reactive oxygen species. In general support of the oxidative damage hypothesis, antioxidants tested in clinical studies have been reported to have a moderate beneficial effect of reducing progression to severe AMD (Hyman and Neborsky, 2002), although the results of several studies are conflicting (Flood et al., 2002). Smoking, which can reduce plasma levels of antioxidants, has been associated with increased risk of AMD (Mitchell et al., 2002). Adding support to the oxidative damage theory is a recent proteomic analysis of drusen, which demonstrated the presence in these deposits of several oxidation-modified products (Crabb et al., 2002).
It has been proposed that dysfunction in the RPE is central to the pathogenesis of AMD and can lead to drusen formation (Hogan, 1972). The earliest known sign of RPE dysfunction is accumulation of lipofuscin, which may lead to the characteristic thickening of Bruch's membrane, formation of drusen, and choroidal neovascularization observed in the wet form of AMD (Gass et al., 1985; Sarks et al., 1988; Green, 1999). Lipofuscin is composed of oxidized, polymeric molecules derived mostly from phagocytosed membranes of photoreceoptor outer segments (OS) (Katz, 1989; Kennedy et al., 1995). OS membranes are known to be rich in polyunsaturated fatty acids, which are an excellent substrate for peroxidation (Katz, 1989). It is believed that these molecules cannot be degraded and therefore begin to accumulate in the RPE cells as lipofuscin. At least one component of lipofuscin, i.e., the fluorophore A2E, a pyridinium bisretinoid, has been demonstrated to be toxic, causing membrane destabilization (De and Sakmar, 2002), and inhibition of cytochrome c oxidase and apoptosis in cultured porcine and human RPE cells (Shaban et al., 2002). Thus, A2E and lipofuscin accumulation in the RPE is thought to be directly related to dysfunction and demise of these cells with aging.
The processes of oxidative damage, lipofuscin accumulation, and drusen formation are not limited to AMD, but rather occur to some extent in all individuals with advancing age. Accordingly, a fundamental question that remains unanswered is why these processes are more advanced in some people than others, leading to AMD. Progress in developing new therapies targeting the root cause of AMD will require much greater knowledge of specific gene targets involved in the key cellular metabolic pathways in photoreceptors, RPE and choroidal cells that contribute to the observed pathology.